Paste X-flipped Pose half working
The orientation of his right arm bones is bad:

In Edit mode select all these bones and ctrl R 180 to rotate 180° on their axis. Then, in Pose mode, go in the Properties panel > Bone Constraints > IK, you'll have to correct the IK Pole Angle -> 180°.

Or, simpler, in Edit mode, select all the left arm bones and go on the bottom menu bar > Armature > Symmetrize. Then, in Pose mode, go in the Properties panel > Bone Constraints > IK, you'll have to correct the IK Pole Angle -> 180°.

Now it works fine.